Large power transformers and high voltage shunt reactors are characterized by their load losses remaining nearly constant with increasing size. For example, a 100-MVA power transformer might have a power factor of 2 percent while another with a 250 MVA rating would have a power factor of 1 percent. Corresponding power factors for high voltage shunt reactors are typically 0.4 percent at 50 MVA and 0.2 percent at 100 MVA. Accurate loss measurements at such low power factors are difficult because of the presence of the large quadrature component of current. For power measurement that is accurate to 1 percent of 0.002 power factor, a technique with an overall accuracy of (0.002.times.1) or 0.002 percent is required. This is equivalent to a total phase error of not more than 20 .mu.rad anywhere in the measurement system, usually in the voltage and current transducers, or if there are no phase errors, an accuracy of 20 ppm (parts per million) of full scale in the wattmeter. Although highly accurate standard wattmeters (G. N. Stenbakken, "A wide band sampling wattmeter", IEEE Trans. Power App. Syst., Vol. PAS-103, No. 10, pp. 2919-2926, October 1984), active voltage dividers (E. So, "The application of the current comparator in instrumentation for high voltage power measurements at very low power factors", IEEE Trans. Power Delivery, Vol. PWDRD-1, No. 1, pp. 98-104, January 1986), and current transformers (W. J. M. Moore, G. Love and F. A. Raftis, "Measurement of short circuit load losses in large three phase power transformers using an alternating current comparator bridge", IEEE Trans. Power App. Syst., Vol. PAS-94, No. 6, pp. 2074-2076, November/December 1975) have been developed, the stringent high accuracy requirements of the instruments can be greatly reduced by removing the large quadrature component of the load current so that the wattmeter would be operating at/or near unity power factor. At unity power factor, accurate knowledge of the voltage and current magnitude is essential but considerable latitude in the phase angle between these two quantities is permissible. For a power measurement that is accurate to 1 percent of unity power factor, a total phase error of up to 0.14 radians or 487 minutes can be tolerated, or if there are no phase errors, a wattmeter with an accuracy of 1 percent is sufficient.
This technique of measuring the loss component of the load current by removing its quadrature component has basically been implemented in the bridge technique for measuring load losses. Such measurements using a current-comparator-based capacitance bridge have been described (see the above-referenced article by Moore et al). The quadrature component of the load current is balanced in the current comparator against the quadrature current obtained from a low-loss high-voltage compressed-gas-dielectric capacitor. The loss component is then balanced by the loss tangent (tan .delta.) balancing circuit, which does not require great accuracy. Its accuracy is only in the order of 0.5 to 1 percent of the tan .delta. reading.
A semi-automatic bridge has been described (R. Malewski, W. E. Anderson and W. J. M. Moore, "Inter-laboratory comparison of EHV shunt reactor loss measurements", IEEE Trans. Power App. Syst., Vol. PAS-99, No. 4, pp. 1634-1641, July/August 1980) in which the quadrature component of the inductive load current is manually balanced, approximately, in the current comparator against the current from the reference capacitor so that only a small residual unbalance current remains. The load loss component of this residual current is extracted by multiplying a voltage proportional to the residual current by a replica of the source voltage. The multiplier output is then proportional to the power loss of the inductive load.